Portable small-sized electronic appliances, such as cellular phones, personal digital assistants (PDA), notebook PCs, and video cameras, have been becoming more and more multifunctional, and the electric power consumed by such appliances and the continuous operating time tend to increase commensurately. To cope with such tendency, there is a strong demand to heighten the energy density of power sources for such appliances. Currently, lithium secondary batteries are mainly used as such power sources, but the energy density of lithium secondary batteries is estimated to reach their limit at about 500 Wh/L or 200 Wh/kg around the year of 2005. It is therefore desired that polymer electrolyte fuel cells (PEFC) be commercialized as early as possible as an alternative power source to lithium secondary batteries. Among PEFCs, direct-type fuel cells, which generate electricity by directly supplying an organic fuel to the anode without reforming it into hydrogen, are considered to be particularly promising. As the organic fuel, for example, methanol, ethanol, dimethyl ether, etc., are used. Direct-type fuel cells have been receiving attention in terms of the high theoretical energy density of the organic fuel, system simplification, the ease of fuel storage, etc., and hence their research and development is actively underway.
A direct-type fuel cell includes an anode, a cathode, and a polymer electrolyte membrane sandwiched therebetween. Each of the anode and the cathode has a catalyst layer in contact with the polymer electrolyte membrane, and a gas diffusion layer disposed on the outer side of the catalyst layer. A mixture of an organic fuel and water is directly supplied to the anode while an oxidant, for example, oxygen or air, is supplied to the cathode. An electrochemical reaction between the fuel and the oxidant produces electricity.
The electrode reactions of a direct-type fuel cell (DMFC) using, for example, methanol as the organic fuel are as follows.
Anode electrode: CH3OH+H2O→CO2+6H++6e−
Cathode electrode: 3/2O2+6H++6e−→3H2O
That is, on the anode, methanol reacts with water to produce carbon dioxide, protons, and electrons. The protons pass through the electrolyte membrane and reach the cathode. On the cathode, oxygen and the protons combine together with the electrons that have passed through an external circuit to produce water. Thus, if the completely ideal reaction takes place on the anode, the chemical substance discharged from the anode is only carbon dioxide (carbonic acid gas). However, since an aqueous methanol solution with a concentration of 3 to 30 wt % is usually supplied to the anode, unreacted methanol and significantly excessive water are discharged from the anode. Further, the effluent discharged from the anode may contain side-reaction products (formaldehyde, formic acid, etc.) or chemical substances that are in an intermediate stage of reaction.
Methanol, formaldehyde, and formic acid are designated as deleterious substances under Poisonous and Deleterious Substances Control Law. Also, since formaldehyde can be a cause of sick building syndrome, the amount of release thereof is strictly limited. According to the Recommendation of Occupational Exposure Limits (2002) of Japan Society for Occupational Health, the allowable concentration of each substance is 200 ppm for methanol, 0.5 ppm for formaldehyde, and 5 ppm for formic acid. The allowable concentration refers to the concentration that is regarded as having no adverse health effect on almost all workers who are exposed to a harmful substance for 8 hours per day, about 40 hours per week. Further, with respect to formaldehyde, the Ministry of Health, Labor and Welfare has drawn up a guideline value of indoor concentration of 0.08 ppm.
When an electronic appliance equipped with a direct-type fuel cell is used indoors for an extended period of time, it is essential to control the concentration of unreacted fuel or by-products in the effluent discharged from the fuel cell at a low level such that it does not exceed the above-mentioned allowable concentration. It appears that solving this problem is a key to commercialization of direct-type fuel cells.
In DMFCs, one method proposed for separating methanol and carbon dioxide that are discharged from the anode is cooling a mixed gas of vaporized methanol and carbon dioxide to condense most of the methanol. Another proposed method is to separate them by using a separation membrane and vent only the carbon dioxide to the outside.
However, in the case of the former method, in actuality it is extremely difficult to mount a mechanism that cools the exhaust gas until the vapor pressure of methanol becomes sufficiently low in electronic appliances equipped with a DMFC. Also, the latter method cannot become an effective method unless a separation membrane with a sufficiently high separation factor of methanol and carbon dioxide and a high permeation speed of carbon dioxide is available. At present, a separation membrane with an excellent function is not available.
Meanwhile, purifying methods and apparatus for combusting methanol, formaldehyde, etc., in exhaust gas by using an oxidation catalyst have been proposed. For example, Patent Document 1, which does not relate to a direct-type fuel cell system, discloses a fuel cell system including a vaporizer for vaporizing methanol and water and a heater for heating the vaporizer through combustion reaction of methanol. It proposes using a trap catalyst for trapping uncombusted methanol and aldehyde discharged from the heater. In order to oxidize the uncombusted methanol and aldehyde, the trap catalyst is supplied with hydrogen or air discharged from the fuel cell.
Patent Document 2 discloses a direct-type fuel cell system using a liquid fuel and proposes the use of a gas-liquid separation bath for separating products of electrochemical reaction into gas and liquid and a filter for absorbing or decomposing by-products contained in the separated gas. Also, a honeycomb layer carrying a noble metal catalyst or the like is used as the filter.
Patent Document 3, which does not relate to a direct-type fuel cell system, proposes a method of deodorizing exhaust gas by oxidizing the small amount of aldehyde contained in combusted exhaust gas by using an oxidation catalyst including manganese dioxide and copper (II) oxide as main components. Also, there has been proposed a catalyst unit for deodorizing combusted exhaust gas in which oxidation catalysts are filled in layers.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-17835
Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-223920
Patent Document 3: Japanese Laid-Open Patent Publication No. Hei 9-206596